1. Field of the Invention
The present invention relates to a development device, a process cartridge that includes the development device, and an image forming apparatus such as a copier, a printer, a facsimile machine, a plotter, a multi-function machine, and the like that incorporates the process cartridge.
2. Description of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer. In electrophotographic image forming apparatuses, two-component developer consisting essentially of toner and carrier particles is widely used.
Recently, downsizing is required in image forming apparatuses using electrophotographic method, such as copiers and printers, for example, for using the image forming apparatus personally, and accordingly, downsizing is sought for development devices in the image forming apparatuses.
In order to satisfy this demand, disposable development devices and process cartridges are widely used. More specifically, the disposable development device can be replaced entirely with a new one after the developer in the development device is consumed. The process cartridge houses the development device, a latent image carrier, such as a photoreceptor on which a latent image of a document image is formed, and a cleaning device that removes the residual toner.
However, the above-described compact development device has a smaller capacity for the developer consisting essentially of toner and magnetic carrier, which means that the agitation space of the developer has to be made more compact. In addition, recently, as the diameter of the toner particles that comprise the toner has become smaller to form better-quality images, it becomes increasingly difficult to disperse the toner that is supplied to the development device into the developer uniformly and to charge the toner uniformly. Failure to disperse the supplied toner into the developer sufficiently can cause the supplied toner that has not been charged enough to slide on surface of the developer. As a result, when the above-described toner is transported to a developing range by a development roller, image failure, such as, scattering in the backgrounds of output images and fluctuation in the image density may be caused. These image failures tend to be significant when the amount of the supplied toner is increased, for example, when documents whose image ratio is high are continuously printed.
In order to solve theses image failures arising from insufficient agitation of the supplied toner, various methods of supplying toner to the development device have been proposed. Thus, for example, in a development device proposed in JP-2008-257213-A, in order to prevent slippage of the supplied toner, the toner is supplied to the development device by submerging the toner into a lower portion of a developer agitation path through a toner transport path. In this example, the slippage of the toner on the surface of the developer can be prevented because the toner goes under the developer.
In another development device proposed in JP-2005-266511-A, the toner is supplied not by dropping the toner directly to a two-component developer transport path, but by transporting the toner to a toner transport path separated from the developer transport path, and the supplied toner is transported by a screw through the toner transport path to the developer transport path. Then, the toner is mixed with the developer in the developer transport path. In this example, the dispersibility and charging ability of the supplied toner can be improved compared with the configuration in which the toner is supplied to the developer by dropping.
However, in these examples, especially when the development device is left for a long period under high-temperature and high-humidity conditions, the toner in the toner transport path more easily coagulates compared with the toner that does not pass through the toner transport path but is directly mixed with the developer. Such toner coagulation causes image failures such as substandard output images, in which toner is partly absent creating white lines, and output images in which the toner coagulation appears as a lump in an output image on the transfer sheet.
In view of the foregoing, there is market demand for a development device capable of preventing toner from coagulating in the developer-containing portion and improving image quality.